Choose and Upload the firmware - either factory firmware, or another suitable uboot image such as DD-WRT, or soon, OpenWRT (trunk)

When this is working you will get a "Device is upgrading the firmware" page which cycles through from 0 to 100%

I had some troubles getting to this point: Firefox 15.0.1 on Linux amd64, hung at the 'Upload' screen. Chrome 21 on Linux amd64 actually uploaded, (as verified by wireshark) and then hung. I could only succesfully complete this procedure using Windows XP with IE8 (!) My experience contradicts what is documented on the DD-WRT website, which states that Firefox only works.